1. Field of the Invention
The invention relates to a lamp control circuit, more particularly to a circuit for controlling operation of an emergency exit lamp that can prolong the service life of the lamp and that can ensure sufficient brightness of the light output of the lamp.
2. Description of the Related Art
Emergency exit lamps are installed in hallways and exits of a building to guide people to safety in the event of an emergency situation. They also serve as back-up light sources in the event of a power outage.
FIG. 1 illustrates a conventional lamp control circuit 1 for controlling operation of an emergency exit lamp that includes first and second lamp units 10, 12. When electric power is supplied by an external alternating current (ac) power source (V1), lamp ballast 14 activates directly the first lamp unit 10 to generate a light output. In the event of a power outage, oscillator 18 converts the voltage output of storage battery 16 to activate directly the second lamp unit 12. The first lamp unit 10 is extinguished at this time. However, while the effect of indicating an emergency exit can be achieved under all conditions with the use of the lamp control circuit 1, the following drawbacks arise during practical use of the conventional circuit 1:
1. In the event of a power outage, the oscillator 18 converts the direct current (dc) voltage output of the storage battery 16 into a high voltage ac output for activating the second lamp unit 12. Power consumption is increased because of the large amount of current that flows instantaneously to the second lamp unit 12.
2. Due to the large amount of current that flows instantaneously to the second lamp unit 12 when the latter is activated, the temperature of electrical components in the vicinity of the second lamp unit 12 is increased and can lead to a reduction in the service lives of the second lamp unit 12 and the electrical components.
3. Because the power consumed by the second lamp unit 12 is relatively large, the power stored in the storage battery 16 is easily used up, thereby leading to a darker light output for the second lamp unit 12 and in eventual failure of the emergency exit lamp.
4. As the light output of the second lamp unit 12 grows darker due to insufficient power supplied by the storage battery 16, the two ends of the second lamp unit 12 turn black and can further shorten the service life of the second lamp unit 12.
FIG. 2 illustrates another conventional lamp control circuit 2 for controlling operation of an emergency exit lamp 20. When electric power is supplied by an external alternating current (ac) power source (V2), transformer circuit 22 and rectifier circuit 24 cooperatively reduce the voltage and convert the power into a dc power signal. Thereafter, oscillator 26 converts the dc power signal into a high voltage ac power signal to activate the lamp 20 to generate a light output. In the event of a power outage, the oscillator 26 converts the power output of storage battery 28 into the high voltage ac power signal to enable the lamp 20 to continue generating the light output. It is noted that the conventional lamp control circuit 2 suffers from the same drawbacks of the conventional lamp control circuit 1 described beforehand. 1. During a power outage, the oscillator 26 converts the dc voltage output of the storage battery 28 into the high voltage ac power signal for activating the lamp 20. Power consumption is thus increased because of the large amount of current that flows instantaneously to the lamp 20. Due to the large amount of current that flows instantaneously to the lamp 20 when the latter is activated, the temperature of electrical components in the vicinity of the lamp 20 is increased and can lead to a reduction in the service lives of the lamp 20 and the electrical components. Furthermore, because the power consumed by the lamp 20 is relatively large, the power stored in the storage battery 28 is easily used up, thereby leading to a darker light output for the lamp 20. Moreover, as the light output of the lamp 20 grows darker due to insufficient power supplied by the storage battery 28, the two ends of the lamp 20 turn black and can further shorten the service life of the lamp 20.